This invention relates to an explosive carrier for the perforation of downhole casing and the penetration of earth formation therefrom, during oil and gas production operations.
In oil and gas operations, perforating through casing using a perforating gun is probably the most important of all completion jobs in cased holes. After a casing is properly placed in a drilled hole, a charge carrier carrying explosive charges are lowered downhole. Charges are fired to effectuate perforations through the steel casing and into the earth formation therefrom, thereby providing communications between the well bore and the desired producing zones.
In conventional charge carriers, the explosive charges are arranged in a spiral configuration. For 41/2" to 51/2" casings, two sizes of charge carriers are commercially available: 27/8-inch and 33/8-inch. To minimize interference between filed charges, the explosive charges in the conventional carriers are spaced at a 60 degrees phasing and at a vertical distance of about 2 inches. Such a conventional configuration results in a shot density of 6 shots per foot. Because of such a limited spacing, a certain extent of interference exists between the fire of shots. Due to the pressure wave generated by neighboring shots, the hole size is often significantly smaller than what could be achieved if no such interference existed.
With a conventional charge carrier, in order to achieve a desired flow rate, the same cased hole often has to be shot twice. The charge carrier is first lowered to the wellbore, and shots are fired. Then the carrier is pulled back to the surface, to reload charges. The charge carrier then is lowered again to the wellbore and shots are refired. Safety could be a serious concern in doing such multiple trip operations. Some explosives may not have been fired and could explode at the surface and cause serious safety concern. Furthermore, because the charge carrier must be lowered twice, this doubles the possibility that the carrier may get stuck in the pipe, requiring laborious fishing jobs. Multiple trips also consume significant rig time, which could be very expensive, especially during offshore operations. If the charge carrier is not properly positioned in the second run, it could end up shooting the same hole twice, getting interference from previously fired shots. Yet, furthermore, a multiple shooting also has the risk of splitting the casing when two shots are fired together.